Image Alpha Background Renders Black when using “Import Images as Planes” 2.0
In your particular case you did not enable 

Use alpha:



In newer versions of blender that box might not be there but you can still select to have transparency as you import the image:



The trasnsparency will only show on Material or Render view, it will not be displayed in texture view.



For more details read the previous answer: 
Image Alpha Background Renders Black when using &quot;Import Images as Planes&quot;